The neuroblastoma clone N4TG1 possesses specific recognition sites for the putative neuromodulators histamine, acetylcholine and enkephalin. These sites are positively coupled with guanylate cyclase: agonist stimulation in the presence of a phosphodiesterase inhibitor causes a dose-related increase in cellular cyclic GMP (cGMP) content. A marked refractoriness or sensitization of this response develops after short-term agonist exposure. In each case this desensitization is specific or homologous and is not mediated by a decrease in either the binding affinity or number of recognition sites. Agonist-induced desensitization of cGMP formation does not, furthermore, appear to be mediated by alterations in the specific activity of either phosphodiesterase or guanylate cyclase. Rather, the marked increase in the activation constant for cGMP formation in desensitized cells indicates that a change in the coupling function(s) between recognition site occupancy and enzyme stimulation has occured. We are currently examining the roles of membrane calcium channels and phospholipid turnover in desensitization of cGMP synthesis in N4TG1 cells.